In general internal combustion engines, such as diesel engines, where lean burn takes place in which an air-fuel mixture having a high air-fuel ratio (lean atmosphere) is burned occupies a large portion of the entirety of the drive region. Therefore, a NOx storage agent (NOx storage catalyst) for storing (absorbing) nitrogen oxide (hereinafter referred to as NOx) included in the exhaust is placed in the exhaust passage in engines of this type, so that the exhaust is purified.
In such NOx storage catalysts, it is necessary to regenerate the NOx storage catalyst by reducing NOx in the case where the degree of NOx storage reaches a state of saturation. The methods for reducing NOx include a method for adding a NOx reducing agent (fuel, for example, light oil) upstream from the NOx storage catalyst in the exhaust passage. In this case, the fuel thermally decomposes and hydrocarbon is generated, and a process for accelerating the reduction of NOx using this hydrocarbon as a reducing agent (NOx reducing process) is carried out.
The exhaust of diesel engines includes, for example, particulate matter (hereinafter referred to as PM) of which the main component is, for example, carbon, soot, and SOF (soluble organic fraction). These components cause air pollution. In order to purify such PM, a particulate filter is placed in the exhaust passage of diesel engines. In addition, exhaust purifying apparatuses where PM included in the exhaust which passes through the exhaust passage is collected by the particulate filter, and therefore, the amount of emission released into the air is reduced, are known. As the particulate filter, diesel particulate filters (DPFs) and diesel particulate-NOx reduction system (DPNR) catalysts, for example, are used.
In the case where PM is collected using such a particulate filter, pressure loss increases in exhaust which passes through the particulate filter when the amount of deposition of the collected PM becomes great, and the particulate filter becomes clogged. As a result, exhaust back pressure increases in the engine, and thus, the output of the engine and fuel efficiency are reduced. As the method for solving this, a process for accelerating oxidation (combustion) of PM on the particulate filter (PM eliminating process) is carried out by raising the temperature of the exhaust though addition of fuel into the exhaust passage (upstream from the particulate filter).
As described above, in NOx reducing processes and PM eliminating processes, which are carried out in order to prevent reduction of the working effects of the catalyst of purifying exhaust, a fuel adding valve is placed in the exhaust passage, so that fuel (reducing agent) is supplied into the exhaust passage. However, the nozzle hole of the fuel adding valve is exposed to the interior of the exhaust passage. Therefore, such substances as soot and SOF included in the exhaust gas deposit and adhere to the nozzle hole of the fuel adding valve. The deposited and adhering substances are exposed to the exhaust at a high temperature and thus caulks. As a result, the nozzle hole of the fuel adding valve becomes clogged. As a method for preventing such clogging of the fuel adding valve, there is a method for lowering the temperature at the distal end of the fuel adding valve by forcibly adding fuel (hereinafter referred to as “addition for preventing clogging”) with a different timing from the addition of fuel at the time of NOx reduction and PM elimination (see, for example, Patent Document 1).
In a map used for addition for preventing clogging, the amount for addition and the intervals for addition are adjusted when the engine is in a normal state, and the temperature at the distal end of the fuel adding valve is stable. Therefore, as shown in, for example, FIG. 9, the following problem arises in a transient state, where the temperature at the distal end of the fuel adding valve enters the distal end high temperature region A (ON region) (OFF→ON).
That is to say, in conventional control of addition for preventing clogging, the amount of addition corresponding to a map (amount of addition for preventing clogging) is simply added before the temperature at the distal end of the fuel adding valve becomes high in the transient state, where the temperature at the distal end of the fuel adding valve enters the distal end high temperature region A. Therefore, fuel efficiency deteriorates as a result of wasteful addition of fuel. This is described concretely as follows. In the transient state, where the temperature at the distal end of the fuel adding valve enters the distal end high temperature region A, as shown in FIG. 10, the temperature at the distal end of the fuel adding valve (the temperature at the distal end of the valve without fuel added) does not increase immediately, but increases after a certain period of time has elapsed. Therefore, in a case where the amount of addition for preventing clogging which corresponds to a map is added at the point in time when the temperature at the distal end of the fuel adding valve enters the distal end high temperature region A (OFF→ON) as carried out in the conventional control, the amount for addition increases before the temperature at the distal end actually rises, and fuel of an amount corresponding to the hatched portion in FIG. 10 is wasted.
Patent Document 1: Japanese Laid-Open Patent Publication 2003-222019